My query is this...... I have an old canister filter which simply filtered water form my Discus days and now in the shed. If I was to fill this with media for PO4 removal would it still work the same? After all water is pumped in through media and then back out.......

The theory is the same as using a specific phosphate reactor, pump water through some media and let it do it's job. In general, though, canister filters provide too much flow (gallons per hour) to allow the phosphate media to do it's job efficiently. Depending on the media you decide to use, obviously, but most of the widely used media wants to be at a slower flow rate, down around only 100gph or less, as where I imagine your old canister filter has probably several times that in more flow.

Will it still work? Probably, but it will be much less efficient and take much longer for it to make any difference in your system because the contact time between the water and the media is so low.

There are a few phosphate reactors available you should look at, they're quite cheap, as they're essentially just a tube to hold the media and pump the water through. Using an external pump, such as a maxijet 600 (I use a 400) and a gate valve you can control the flow rates better.

__________________

__________________
~Neilan
In the DC Metro Area? Check out GWAPA and WAMAS

Yeah, you want to control the flow level to allow more interaction time between the media and the water. Depending on your canister filter, I don't know if Id try it there, because you may have to choke it down 75+%, and the filter pump may not like that for too long.

__________________
~Neilan
In the DC Metro Area? Check out GWAPA and WAMAS

Also depends on the media used and construction of the canister since some of the older generations (Eheim for example) allow for maximum penetration (water has to flow through central column) whereas other models/brands may allow for water to escape through the sides.

I have thought about reducing down the flow rate but as you mention the pump will be designed to run at a certain rate and governing it down will put undue strain on the motor and I feel eventually burn it out in 6 months................. it may more cost effective to buy a new reactor and media as oppose to trying to go the cheaper option and ruin the media etc........... many thanks for the advice to all guys and girls out there!

__________________"whoever said the pen is mightier than the sword never encountered automatic weapons"

Choking down a centrifugal pump does not put more strain on the motor. In fact it does just the opposite. By slowing down the flow out of a pump, the pump is actually doing less work and will use less current.

Lots of people like using media like phosban. In my little tank it's success was less than worth the price of the phosban. Unless you have phosphate in your water, I would consider the PWC route.